Popcount is... ridiculously standard. Even GPUs offer 64-bit popcount. Bit-reversal is surprisingly useful in my experience as well (ARM and GPUs offer single-cycle bit-reversal). So I'm surprised to hear that RISC-V doesn't have popcount standard.
Bit-reversal is great because adder-circuits carry only goes in one direction. So all of the "least-significant set bit" tricks that are done can be inverted into "most-significant set bit" rather easily with just a bit-reversal. x86 is missing out on bit-reverse, while all other platforms (GPUs, ARM, and Power9) seem to have it.
I'm also of the opinion that multiply-xor-bitreversal-multiply is a very powerful hashing tool (multiplication by odd numbers is 1-to-1 bijective, xor is 1-to-1 bijective, bitreversal is 1-to-1-bijective... so a multiply-xor-bitreverse-multiply cycle can transform any number into a singular 'random' number across all 64-bits space). x86-fans don't get bit-reversal, but bswap can be used to largely the same effect for hashing.